Crustal deformation propagation

Abstract

During the contract period the main emphasis was on the observation of crustal tilts associated with earthquakes, their directions, amplitudes and propagation velocities. Data from the seismology laboratory’s three borehole installations at GLM, PAX, MCK, the COL World Wide Standard Station and the ALPA (Alaska Long Period Array) have been analyzed for tilt steps associated with local earthquakes. Tilt steps from distant earthquakes observed at GLM, PAX and MCK and data presented in the literature are discussed. Observed tilt steps cover a distance range from 8 to 12600 km and a magnitude range from 2 to 8. Tilt step propagation velocities have been observed from 1.2 to 4.0 km/sec, and those corresponding to arrival times of teleseismic S waves. Tilt directions, amplitudes and velocities observed at several stations simultaneously for the same earthquake are internally consistent and are likely to depend on the tectonic environment of the observing station in addition to the focal mechanism. The low velocity of 1.2 km/sec is consistent with a plastic wave propagation. Other velocities are close to the Rayleigh wave short-period group velocity. The tilt amplitude for fixed epicenter distance, focal depth and stress system depends logarithmically on the magnitude, with a possible cut-off, but are too large when compared to existing theoretical dislocation models for a single layered homogeneous isotropic elastic crust. Alternative solutions in terms of more realistic crustal layering and including plastic deformation under preexisting tectonic stress probably could account for a number of observations. The operation of a number of short-period telemetry stations has been discontinued, new ones installed and one was relocated. A seismicity map was published for the period 1968-1971, based on recordings from the telemetry net and the results related to the geology of Alaska. The map and its geology related discussion proved very useful to a number of state agencies (such as the Highways Department) and aroused much interest from private sources as well.